As known, the moulding technology called “rotational” is dedicated to the production of hollow manufactured articles starting from various polymers. The rotational moulding technology provides the use of a specific mould in which a predetermined amount of liquid or powdered plastic is arranged. After the mould is closed, it is rotated about two perpendicular axes and heated, so that the hot material stratifies on the walls of the mould itself. Once the process has ended, the mould is cooled and, after reopening, the moulded piece or manufactured article is removed.
In many moulded pieces or manufactured articles there is the need to apply, during the moulding step, one or more inserts configured to face from the surface of the manufactured article itself. Generally, these inserts are manufactured with a different material (typically metal) from that of the moulded piece and must be perfectly coated with the polymer, as well as be firmly anchored to the moulded piece. An example of an insert can consist of a threaded component that allows the manufactured article to be fixed to other parts of the machinery in which such a manufactured article is intended to be inserted.
One of the conventional techniques for carrying out the co-moulding of the inserts consists of holding the insert against the inner surface of the mould by means of a mechanical fixing device, called insert-holder. The insert-holding device can be made, for example, in the form of a threaded pin that, once inserted through a through-hole of the mould, reaches the inner side of the mould itself and on which the insert is fixed, for example screwing it.
Again according to the prior art, the insert-holding device is usually provided with a system that allows the insert to slide inside the mould when, due to the shrinkage effect typical of the polymer during the solidification step, the manufactured article or piece being moulded tends to detach from the inner surface of the mould and to move away from the surface itself. In this way, the insert can move with the manufactured article, preventing it from “slipping out” during the moulding step. A system frequently used consists in making a bush at the base of the insert-holding device and in interposing a spring between the insert-holding device and the bush itself, so that the insert is held against the mould but can also move without too much effort, compressing the spring.
A typical drawback encountered in moulds provided with insert-holding devices is linked to the possible formation of the so-called “blowholes” around the co-moulded inserts. The blowholes are generated when a gas (air) passage is created through the polymeric material that is still in the molten state and that covers the insert or is close to the insert itself. This possible air passage creates cracking in the manufactured article and thus involves the need of discarding the manufactured article itself that must be remoulded, with an obvious waste of time and money.
This possible air passage is generated by a pressure difference between the inside and the outside of the mould. Such a pressure manages to “release” through the hole used to mount the insert-holding device. This possible air passage can also be due to the expansion of the air (as a result of the temperature change which the mould is subjected to during the rotational process) that remains trapped in some empty area created in the system for fixing the insert with the relative insert-holding device and/or the bush.
Another drawback encountered in moulds provided with insert-holding devices is linked to the scarcity of polymeric material that manages to “coat” the insert itself. In the rotational moulding the coating of the insert, as well of the entire mould, is linked to the temperature of the inner surface of the mould (and of the insert) that, when reaches the melting value of the polymer, starts holding the polymer that adheres thereto and melts. It is therefore important to ensure that the insert reaches such a temperature when inside the mould there is still sufficient polymer to be melted and, even better, that such a temperature is reached preferably earlier in the area of the insert with respect to the rest of the mould, so as to ensure the perfect coating of the insert itself. On the other hand, the area in which the insert is applied necessarily has a greater mass than that of the rest of the mould (due to the presence of the insert-holding device and of the relative bush) and, moreover, the insert itself typically has a reduced contact surface with respect to the mould, which receives a greater amount of heat from the hot air oven inside which it is arranged. Consequently, the insert-holding device is often penalised in receiving the heat irradiated by the oven.
In systems according to the prior art, the moulds for the rotational moulding are indeed introduced into hot air ovens and their heating takes place by convection. This classical solution makes it complex to manage the heating of the mould in a differentiated manner, for example favouring the heating of certain areas of the mould. Moreover, due to the high temperature of the oven (between 280° C. and 350° C.), it is also technically difficult to apply external elements to the mould that allow the temperature to be detected, as well as the application of circuits through which it is possible to manage and control the internal pressure of the mould.
In the rotational moulding technology it is known that advantages can be obtained, in certain steps of the transformation, either by applying a pressure inside the mould, or by creating a certain degree of vacuum. These provisions allow better compacting the polymeric material through the elimination, during sintering, of possible air bubbles that can form inside the material itself.
In the case in which action is taken on the pressure/vacuum inside the mould, it is however necessary to introduce technical solutions that avoid the possible defects that the pressure and/or vacuum can create. Indeed, at the communication points between the inside and the outside of the mould the pressure difference across the molten polymer leads to the cracking of the manufactured article, moving the material in such an area and generating a waste.